Patent document 1 has disclosed a conventional dry clutch. The dry clutch includes a first piston, which is movable in a cylinder formed in a motor cover under oil pressure supplied, and a second piston, which is installed on the first piston via a bearing interposed between them. Engagement and release of the dry clutch is performed by operating the second piston by means of sliding motion of the first piston. The reason for the provision of the bearing is that the first piston is accommodated in the motor cover, serving as a stationary wall, whereas the second piston serves as a piston brought into direct-contact with a side face of one of clutch plates through which connection and disconnection of rotors can be made, and thus relative rotation between the first and second pistons must be permitted.    Patent document: Japanese Patent Provisional Publication No. 2006-137406 (A)